I wonder if this could be somewhat related to Pxelinux, I am able to successfully netboot the troublesome box using a separate PXE server running Pxelinux version 4.06. Apparently after versions >5.0 some file dependencies are introduced for pxelinux.0 and the c32 modules (and they are arch specific also since version 6.0).
Noseying around in /var/lib/maas/boot-resources/current/bootloader/ there does seem to be several version of the boot resources but they differ from the docs (or at least appear to).
This may be something that’s already handled within MAAS code but I haven’t delved in to see. I’m posting this here in case it hadn’t been considered 